Method of manufacture



uy 2L, 1942@ R. P. KQEHRHNG METHOD OF MNUFACTURE Filed Feb. 28, 1941 fi do v .Patented Y July (21, Y '1942' UNITED- STATES PATENT OFFICE METHOD F MANUFACTURE amano P. xoehring, Dayton, ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Appumiiiretmary 2s, 1941, serial Np. 381,185

y 8 Claims.

, This invention relates to a method of making composite metal articles andis particularly concerned with a method of continuously making composite articles which include a porous metal layer thereon.

An object of the present invention is to provide a method of continuously making composite articles which include a. strip of strong metal such as steel that has a. layer of porous metal bonded thereto wherein the sintering operation is carried out by means of electrical heating.

A further object is to provide a method whereby powdered metal supported on a strong metal backing may be sintered and bonded to the backing by induced electrical currents or by resistance heating of the backing.

Another object is to provide a method whereby a single metal powder, for example, oxygen-free copper powder, can be sintered to the surface of the steel strip wherein the thin layer of powder immediately adjacent the strip is melted while the remainder of the powder layer is sintered together to form a porous copper layer.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein preferred embodiments of the presentinvention are clearly shown.

In the drawing:

Fig. 1 is a diagrammatic view of the induction heating apparatus for bonding and sintering porous metal to the surface of the steel strip;

Fig. 2 is a diagrammatic view showing the apparatus for sintering and bonding a porous metal layer by means-of the resistance heating of the strip;

Fig. 3 is a, section taken through line 3 3 of Fig. 2:

Fig. 4 is a section taken on the line of Fig. 1. Y

In Patent No. 2,198,253, assigned to the assignee of the present invention I have disclosed the method for making composite strip material in a continuous manner which strip includes a strong metal supporting back, for example, steel upon which is sintered a porous metal layer made from loose non-compacted metal powder which layer is metallurgically bonded to the surface of the strip. Inf the method disclosed in this patent the sintering and bonding is carried out within a furnace preferably heated by means of which is particularly de-A y The present invention is directed to another `method of sintering and simultaneously bonding powdered metal upon the surface of the strong metal strip wherein the length of the sintering `5 operation is materially shortened and wherein it is possible to sinter and bond a single metal powder tothe surface of a strip without markedly reducing the porosity of the powdered metal layer. l0 It has been found that bearings which include a porous metal layer of oxygen-free, high conductivity copper on the surface thereof are particularly desirable. Likewise atomized copperlead particles which have been sintered and bonded to a strip of steel make excellent bearings. In this instance the copper and lead which do not alloy appreciably are segregated in each particle. It is further desirable to have a. bronze layer sintered to steel which layer may be formed from a mixture of copper and tin powders, or may be formed from bronze powder which has been made from previously alloyed copper and tin 'that has been comminuted from ingot form, or in some cases it is desirable to form a layer of copper nickel alloy either by using mixtures of copper nickel powders or by using comminuted copper nickel alloy. In the case of the alloy powders it has been found desirable in most instances to utilize powders which are not completely alloyed, that is to say that if a bronze is desired it is preferable to have powder particles having a tin-rich surface with a copper rich nucleus. This distribution of components aids the sintering operation since there is still appreciable diifusion upon sintering whereby the tin diffuses into adjacent particles and forms a stronger bond. Powder from completely alloyed metals falls in the same class as single metal powder since the powder has a xed melting point.

I propose to utilize electrical energy as the heating medium for accomplishing the sintering and bonding steps. This energy may be in the form of induction heating or may be in the form of heat derived from resistance heating wherein the resistance utilized is preferably of the steel strip itself.

Referring particularly to Fig. 1 an induction heating apparatus is shown diagrammatically wherein a steel strip 2U passes in the direction of the arrow through an induction heating coil 22.

',I'hisstrip 20 is supported by a. plurality of rolls 24 and the coil 22 is preferably enclosed by means of a casing 26. Metal powder 28 held in a hopper 30 is fed by the aid of gravity upon the surface of the strip 3| and is smoothed into a substantially non-compacted layer by means of a smoothing device 32. Strip 20 with the metal powder layer thereon then passes through the heating coil 22 and is heated by means of the induced curo rent from the coil which is supplied from a suitable current source. The rate of heating is extremely rapid and the sintering and simultaneous bonding of the powder metal layer is accomplished in a most expeditious fashion. After passing through the coil 22 the strip 20 with a porous metal layer 3B thereon next passes between pressure rolls 28 if desired to compact the porous metal layer to any degree oi' density whereby the porosity thereof is controlled. The apparatus as described is suitable for use with copper-tin powder, copper-nickel powder and in fact any combination of metals wherein the melting point of the component metal diii'ers appreciably.

When sintering oxygen free copper powder or other single metal powders to the surface of the strip, it is desirable to have high localized temperature at the exact surraceoi thestrip whereas Y the remainder of the porous metal layer is preferably held below the melting point of the copper. In other words if the entire layer of copper powder is heated above the melting point it is obvious the layer will melt and destroy the porosity of the layer. However. if a microscopic layer of copper powder immediately adjacent the steel strip is melted, it is possible to sinter together particles of copper in the large mass of powder layer to form a porous Ycopper layer and simultaneously melt sufficient copper to braze this porous metal layer to the steel. In some cases, it may be desirable to place a layer of copper powder or the like upon the surface oi a steel strip or plate wherein the layer is vonly one particle thick. In this instance it is probable that the mesh size of the powder would be relatively large. In this instance it may be desirable to heat the portion of the copper particles adjacent the steel strip' to reach the melting point but maintain the greater portion of each particle below its melting point simultaneously, or it may be desirable to place a layer of ne mesh copper powder at the surface of thev strip `superimposed thereon a layer of coarse mesh powder. The differential heating is accomplished by the use of a water cooled chamber 40 which is disposed immediately adjacent and on top of the powder metal layer. The-chamber 40 removes suicient heat from the upper portion of the powdered metal layer to prevent the temperature exceeding the melting point of the metal powder whereas the metal powder immediately adjacent is heated in excess of its melting point and therefore flows over the surface of the steel and bonds thereto and simultaneously bonds to the remainder of the copper. It is preferred in carrying out these operations to provide a non-oxidizing atmosphere within the casing 26, such atmosphere may be introduced by means of a tube 42 at the entrance end of the furnace which provides a suitable controlled atmosphere which in the case of the oxygen-free copper is preferablyv of a reducing nature but which in the case of other metal powders may be of a non-oxidizing nature. Atmospheres .of this character are well known in the art. When sintering and bonding oxygen-free copper powder it is desirable to adjust the heating by induced current so that the temperature immediate adjacent the surface of the strip exceeds 1981 lli'.. the melting point of the copper, while the remainder of the copper powder layer is maintained in the neighborhood of 1700to 1950 F.

'I'he water cooledv chamber 40 is not entirely necessary wheny sintering mixtures of copper tin powder, copper nickel powder and the like, wherein sintering temperatures of from 1400 to 1700 and from 1900 to 2100 respectively are desirable. Obviously, these sintering temperatures may be increased so that an actual melting of all component metals takes place immediately adjacent the surface of the steel strip, by the introduction of the water cooled chamber 40. If the temperature is properly chosen, the large mass of powdered metal layer is not melted but is only heated above the melting point of one of the constituents.

Another method of accomplishing similar results is the use of the steel strip per se as the heating medium. In this instance the strip 20 passes under the hopper 30 and as the metal powder. 28 distributed thereon is smoothed out by the device 32 the strip with the metal powder thereon then passesrthrough a pair of spring- Y pressed rollers 50 and 52 respectively. It will be noted from Fig. 3 that the metal powder is preferably conilned to the center portion of the steel strip whereby the rollers 50 and 52 may engage the edge of the steel strip. Obviously if the powdered metal layer 2| is spread over the entire width of the steel strip 20 the rollers 50 and 52 must Contact the edge of the strip which can be done by bevelingthe rollers and placing them so as to contact only the edge. In any event, a pair of spring-pressed brushes 54 and 58 are associated with rollers 50 and 52 respectively whereby the brushes are pressed into wiping relation with the surface of the strip 20. In this manner, when the brushes are connected to a suitable current source, current is passed into brush 50 through the strip 20 and to brush 56 which is connected to the other side of the current source to complete the circuit. Suiilcient current is passed through the strip to heat the same progressively as the strip passes between the brushes to a temperature suitable for sintering and bonding the metal powder to the surface of the strip. The portion of the strip between the brushes is preferably enclosed by a casing 58 into which reducing atmosphere may be introduced by means of inlet 60.

A similar heating step may be accomplished in this apparatus as that described in connection with Fig. 1 wherein the temperature immediately adjacent the surface'of the strip 20 can be maintained at a higher degree than the temperature within the main mass of powdered metal layer by use of a water cooled chamber 62, which is placed immediately adjacent the upper surface of the powdered metal layer. As previously ex- Power required in watts =68.7dlA Sp. Ht. T- 26C.) y

The current can be found directly without having to find the power required in watts thusly:

1:17am sp. Ht. (iT-26C.)

tr,(1 +aT) Meanin'g of symbols d-density of the strip in grams per cubic centimeter.

l-the distance between the contact brushes in inches.

A-cross sectional area of the strip in square inches.

Sp. Ht.-Specic heat of the strip in calories per degree centigrade per gram (average).

T-temperatureto which strip is to be heated in degrees centigrade.

t-time required for a point on the strip to pass from one contact brush to the other.

ro-resistivity in ohms at zero degrees centigrade of a unit centimeter cube.

a-temperature coeicient of resistivity.

In either of the methods disclosed herein greater speed of lheating is obtained whereby the strip may be continuously passed through the electrical heating means whether it be an inductioncoil or a, pair of brush resistance heating of the strip, wherein the metal powdered layer may be simultaneously sintered together and bonded to the surface of the strip.

The methods disclosed herein are particularly adaptable to the formation of continuous composite materials and sintering furnaces heretofore necessary may be eliminated or if desired, the entire induction heating apparatus or resistance heating apparatus may be placed within the usual type of furnace wherein the temperature is maintained, for example, 100 or 200 degrees below the desired sintering temperature. thereby preheating the stock and powdered metal layer thereon and wherein the last 100 and 200 degree rise in temperature is accomplished by v It is apparent that similar heating steps may be carried out on metal powder which is not in the loose non-compacted condition but which has been previously briquetted upon the surface of the steel strip, such procedure being clearly shown and described in Patent No. 2,158,461, assigned to the assignee of this invention.

While the embodiments of the present invention as herein disclosed, constitute preferred forms, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow.

What is claimed is as follows:

1. In a method of sintering and simultaneously bonding a layer of powdered metal to the surface of a steel strip or the like. the steps of progressively electrically heating a portion of the strip with the metal powder layer thereon under suitable conditions for causing the metal powder layer to sinter into 4a porous metal layer and simultaneously bond the surface of the steel strip by causing heat to be generated within the strip by means of the electrical currents therein.

2. The method as claimed in claim 1 wherein the electric heating is accomplished by means of an induced current.

3. The method as claimed in claim 1 wherein the electric heat is accomplished by means of resistant heating of the steel strip.

4. The method as claimed in claim 1 with the additional step of cooling the major portion of the metal powder layer below the temperature of the metal powder layer immediately adjacent the strip whereby a thin layer of the metal powder layer is melted while the remainder of the layer is sintered together.

5. In a method of continuously making com'- posite metal articles, the steps of continuously distributing a'uniform layer of non-compacted metal 'powder upon the surface of the steel strip or the like, passing said strip with the metal powder thereon through an induction coil for `heating the strip with the metal powder layer thereon by meansof induced electric currents to cause the metal powder to sinter together into a strong porous metal layer which is simultaneously bonded to a steel strip.

6. 'I'he method of continuously making composite strip material, the steps of, continuously distributing a uniform layer of non-compacted metal powder upon the surface of the steel strip or the like, progressively heating a portion of the strip by means of the electrical resistance of the strip, said heating being suilicient to cause the metal powder layer to sinter together into a porous metal layer and simultaneously bond the surface to the steel strip.

7. 'I'he method of continuously making composite strip material including a porous metal layer of oxygen-free copper bonded to the surface of a steel strip or the like comprising the steps of, applying a layer of oxygen free copper powder upon the surface of the steel strip, progressively heating a portion of the strip electrically to a temperature slightly above the melting point .of the copper for melting the copper immediately adjacent the surface of the strip, and simultaneously cooling the copper powder layer that is not immediately adjacent the surface of the strip slightly below the melting point of the copper whereby the copper powder layer is sintered together into a porous copper layer which simultaneously bonds to the surface of the strip, all of said steps being carried out under suitable atmospheric conditions.

8. In a method of sintering metal powder to the surface of steel or the like, that step of differentially heating a layer of metal powder to a temperature above its melting point immediately adjacent the surface of the strip and to a temperature below its melting point in that portion which is not immediately adjacent the strip whereby the metal powder layer is sintered into a porous metal layer which is bonded through the medium of molten metal layer formed from the metal powder to the surface of the steel strip.

ROLAND P. KOEHRING. 

